The majority of the casings of steering wheels with integrated airbag are constructed today in two parts, with a shaped part surrounding the steering wheel skeleton and with a separately produced covering for the airbag installation opening in the hub region of the steering wheel, great efforts being made to match the two parts with each other in their visual appearance and haptic characteristics. Considerable further difficulties result from the fact that the airbag covering must be constructed so as to be able to be torn open and so as to be movable relative to the steering wheel hub about the touch path necessary to actuate the horn.
As a gap between the shaped part surrounding the steering wheel and the airbag covering is regarded as visually disadvantageous, it has already been proposed to produce the shaped part and airbag covering in one piece. Owing to the tear-open- and actuating functions to be taken into account in the region of the airbag coverings, such methods present certain substantially solvable difficulties with regard to manufacturing technique and automatically require a largely new design of the steering wheel construction, because assembly can only take place from the reverse side of the steering wheel and because special precautions have to be taken for the introduction of the airbag module with folded gas bag and gas generator. Rather, one has found a way to divide the steering wheel skeleton into a base part with hub and possibly spoke stumps, which can be equipped with the airbag module and other function parts, before the steering wheel rim with spokes or spoke stumps and covering in a single piece is put in place and connected with the base part.
It is regarded as particularly disadvantageous here that the fastening between the base part and the steering wheel rim is connected with an additional expenditure of material, which goes against the general aim of saving weight in vehicle construction. In addition, it is difficult to match the two components optimally to each other with regard to size. In particular, the folded gas bag is equipped as a shaped part with considerable tolerances, so that the coordination of the covering region is not simple, which--as it has to be able to be torn open--must not have any particular inherent rigidity and therefore requires the folded gas bag as a support. If the gas bag set is too "thin", the covering is not sufficiently supported; if it is too "thick", it is buckled upwards. Both are undesirable.
Similar problems occur with regard to the actuating movements which have to be transferred from the exterior over the plastic shaped part constructed in one piece on the steering wheel rim onto the actuating members mounted in the base part. It should be readily clear that the adaptation of a surface produced by casting mold technique to a mounted surface is very costly in terms of manufacturing technique and that the transfer functions can only be carried out with reliable operation when very close tolerances can be maintained in the adaptation.